Automobile bodies and many industrial and consumer articles are conventionally spray painted in areas called spray booths, wherein water curtains are employed to cleanse the air of over-sprayed paint. The wash water is then treated to remove paint solids, and the treated water is recirculated.
A water curtain is typically created by pumping water into a trough above the overspray area. The overflow from the trough is controlled so that a uniform sheet of water falls along the length of the trough and down the booth's wall. Spray nozzles are also commonly used. Fine droplets of over sprayed paint, emitted by a spray gun, contact and are captured by the water curtain. The amount of paint contacting a water curtain may change depending on a number of variables, including plant or process shutdowns, the size and shape of the object being painted, the type of spray equipment used, the spraying and purge technique used, and the water flow and the type of paint used.
"Paint," as used herein, is a generic term which encompasses all common varieties of both water-based and solvent-based paints. Solvent-based paints have commonly been employed in spray booths, but in response to federal regulations limiting the amount of volatile hydrocarbons, namely the solvent diluent used in solvent-based paint, that can be emitted from a plant site, water-based paint is now also being used in spray booth operations.
The term "water-based paints," as used herein, refers to all varieties of coatings which contain in excess of approximately 10% water in the coating formulation, including, but not limited to, water-reducible alkyl and epoxy ester compositions, water-borne thermoplastic latex compositions using acrylic polymer/copolymers, water-based latexes of polyurethane dispersions, and blends of such compositions. As used herein, the terms "water-based paints" and "water-borne paints" are synonymous.
The term "solvent-based paints," as used herein refers to all common varieties of water insoluble coatings including but not limited to oil base paints, enamels and lacquers. As used herein, the terms "solvent-based paints" and "solvent-borne paints" are synonymous.
A primary treatment objective relative to both water-based and solvent-based paints is to capture and collect finely dispersed paint solids. A major problem associated with spraying operations concerns the tacky or adhesive nature of the over-sprayed coating materials. Solids tend to agglomerate and accumulate on the walls, ceiling and floor of the spray area and to clog water spray equipment, recirculating pumps and the like. Paint that sticks to spray booth surfaces usually cannot be easily removed from the equipment and tends to build up over time, which hampers spray booth efficiency. Thus, the overspray, or paint mist, captured in the water system of a spray booth must be detackified, or "killed," before it adheres to the walls, piping, etc., of the spray booth. The term "paint killing" as used herein refers to the act of detackifying oversprayed paint in paint spray booths.
Other problems which severely interfere with spray booth operations occur in areas of high agitation where foaming occurs and in areas where foam accumulates. Foaming is caused by chemical additives, surfactants, solvents or combinations thereof. Finely dispersed paint solids which are not captured and removed tend to stabilize foam, which aggravates foaming problems. Foaming generally mandates that copious amounts of defoamers be used, which results in higher operating costs. Water-based paints generally tend to cause foaming to a greater extent than solvent-based paints.
A wide variety of chemicals have been proposed as treating agents for circulating wet spray booth waters containing overspray paint, including compositions containing polymers and amphoteric metal salts which form insoluble hydroxides at pH's greater than about 7. The use of combinations of this type are described in the following U.S. Pat. Nos.: 3,861,887 to Forney; 3,990,896 to Gabel et al.; 4,002,490 to Michalski et al.; 4,130,674 to Roberts et al.; and 4,440,647 to Puchalski. Further, U.S. Pat. No. 4,637,824 to Pominville discloses the use of silicates and polydiallyldialkylammonium halides with amphoteric metal salts, and U.S. Pat. No. 4,853,132 to Merrell et al. discloses the use of precipitates formed by the reaction of cationic polymers and salts of inorganic anions to detackify solvent-based paints. Bentonite clays, aluminum salts and zinc salts have also been used with cationic polymers.
U.S. Pat. No. 4,656,059 to Mizuno et al. relates to the use of melamine-aldehyde acid colloid solutions for treating paint in wet spray booths, and U.S. Pat. No. 4,629,572 to Leitz et al. relates to the use of urea or amino triazine-aldehyde condensation reaction products in combination with water-swellable clays to treat paint spray booth wastes.
The instant invention represents an alternative to the inventions of the prior art in that an alumina coated silica sol, used in conjunction with aqueous polymer flocculant, can be used to treat circulating paint spray booth waters containing over-sprayed paints.
Methods of preparing alumina coated silica sols are taught by DuPont in the following patents: U.S. Pat. Nos. 3,007,878; 3,719,607; and 3,956,171. These DuPont patents stress the stability of the claimed alumina coated silica sols but do not discuss or claim applications thereof.